The present invention relates to expansion joints for use with electrical conduits and, in particular, to the electrical grounding of such expansion joints.
It is conventional to run electric wiring within a metal conduit and to electrically ground the conduit to protect persons who may come into contact with the conduit. In certain instances it may be desirable to form the conduit as separate sections interconnected by a thermal expansion joint for permitting relative movement between the conduit sections in response to ambient temperature fluctuations. It then becomes necessary to maintain the continuity of the electrical ground across the expansion joint.
A conventional expansion joint 10 depicted in FIG. 1 is connected to first and second sections 12A, 12B of a metal conduit for electrical wiring (not shown). The expansion joint 10 comprises a metal reducer 14 having one end threaded onto an end of the first conduit section 12A, and another end threaded to a cylindrical metal sleeve 16. Threaded to an opposite end of the sleeve 16 is a metal gland nut 18 which encompasses a fiber gasket 20, a steel washer 22 and a packing 24. The packing may comprise an asbestos and copper braid and functions to resist the entry of moisture into the interior of the sleeve 16. The second conduit section 12B passes through the packing 24, and the packing is compressed against the second conduit section 12B by tightening the gland nut 18. A plastic throat bushing 26 is screw-threaded onto the end of the second conduit section 12B to provide a smooth inner diameter against with the wires can engage.
Linear movement of the second conduit section 12B relative to the expansion joint 10 in response to thermal variations is possible since the second conduit section is slidable relative to the packing 24.
In order to maintain electrical ground continuity from the conduit section 12A to the conduit section 12B, a grounding device 30 is connected between the conduit sections 12A, 12B. That grounding device comprises a metal grounding strap 32 connected to respective conduit sections 12A, 12B by means of clamps 34A, 34B. Such a grounding device is relatively expensive and bulky and is susceptible to environmental and/or handling damage since it is entirely exposed.
Another type of grounding device proposed in U.S. Pat. No. 3,154,632 comprises a braided copper ring positioned around the outer circumference of a portion of the conduit section located within the gland nut. The copper ring is located axially between the packing and the end of the gland nut. When the gland nut is tightened, the copper ring is wedged into contact with the outer periphery of the conduit section by the gland nut and thereby maintains the electrical ground. In practice, however, thermally-induced relative movement between the expansion joint and that conduit section results in the specially-applied outer plating of the conduit section being frictionally abraided away by the copper ring. The conduit section located beneath the abraided region of the plating is then exposed to the ambient environment (e.g. rain, etc.) as the conduit section moves out from the gland nut (during thermally-induced expansion/contraction).
Proposals have been made for providing an electrical ground by means of a ring shaped grounding device mounted on the outside of a cable and situated inside of a connector. Attention is directed for example to U.S. Pat. No. 4,213,664 which discloses a grounding device in the form of a garter spring disposed within a connector in encircling relationship to a cable. The garter spring is sandwiched between an inclined cam face and a compression ring of the connector. In order to cause the garter spring to contact the cable, portions of the connector are moved longitudinally relative to one another to converge the cam face toward the compression ring in order to force the garter spring radially inwardly toward the cable. Provision is made to enable a user to look through the connector and visually inspect the garter spring in order to ensure that a proper grounding connection has been established.
It would be desirable to enable a grounding device to be mounted without requiring a special manipulation of a connector or visual inspection, and without risking abrasion of the outer plating of a conduit section. Also, it would be desirable to provide a grounding device which can be easily retrofit on existing expansion joints.